1. Field of the Invention
The present invention relates to method and apparatus for adjusting a bias current flowing through a magneto-resistive effect type magnetic head to an optimum value and a magnetic recording apparatus having the magneto-resistive effect type magnetic head.
2. Description of the Prior Art
As a reproducing head for a magnetic disk apparatus in which a large volume of high density information is processed, a magnetic head having a magneto-resistive effect device in which an electric resistance changes with a strength of a magnetic field has been used. As shown in FIG. 14(A), for example, a magneto-resistive effect type magnetic head (hereinafter, called an MR head) has a structure in which a soft magnetic layer 101, a non-magnetic layer 102 and a magneto-resistive effect layer 103 are arranged in that order to be piled up, a pair of magnetic domain control magnetic layers 106 and 107 are arranged on both sides of the magneto-resistive effect layer 103 and a pair of leads 104 and 105 are arranged on the magnetic domain control magnetic layers 106 and 107. In the above structure, a bias current Is flows through a sense region W of the magneto-resistive effect layer 103 through the leads 104 and 105, a direction of a magnetization M of the sense region W changes with an external magnetic field, and a resistance of the sense region W is changed. A change of the resistance is detected as a change in voltage. As resistance of the magneto-resistive effect layer 103 becomes large, a reproducing output of the MR head becomes larger. In contrast, a sensitivity of the MR head is lowered as the resistance of the magneto-resistive effect layer 103 becomes large.
Therefore, appropriate adjustment resistance in the MR head is required. As one method for respectively adjusting the resistance in the MR head, there is a method for changing a resistance between the leads 104 and 105 by polishing portions of the soft magnetic layer 101, the non-magnetic layer 102 and the magneto-resistive effect layer 103 opposite to a magnetic recording medium. This polishing is performed for planes of the soft magnetic layer 101, the non-magnetic layer 102 and the magneto-resistive effect layer 103 opposite to the magnetic recording medium, and a resistance of each of the layers 101 to 103 becomes larger as a height of each of the layers 101 to 103 becomes low. In this case, the height each of the layers 101 to 103 is defined as a height in a direction perpendicular to an upper surface of the magnetic recording medium.
However, it is troublesome that the resistance in the MR head is adjusted by polishing the layers 101 to 103, and there is a problem that the resistance cannot be adjusted with a high accuracy.
Also, there is another problem that a life time of the MR head is shortened because of the occurrence of migration in the MR head. The migration is a phenomenon that the magneto-resistive effect layer 103 or the like is melt and cut out by the bias current.
Because the uneven polishing of the layers 101 to 103 is inevitably performed and the migration inevitably occurs, the optimization of a current (or the bias current) flowing though the MR head is performed. To optimize the bias current, as shown in FIG. 14(B), a curved line of the life time is initially determined in rectangular co-ordinates expressing the relationship of the resistance in the MR head (called an MR resistance) and the MR bias current. The curved line of the life time is obtained according to experimental results.
When the bias current is determined according to the curved line of the life time, an upper limit is set for the MR resistance, and the bias current is determined on condition that an MR head in which the MR resistance exceeds the upper limit is not used. Thereafter, a maximum bias current J.sub.0 which is allowable for an MR head in which the MR resistance is equal to the upper limit is determined as a bias current set for all MR heads.
Other methods for respectively determining the bias current are described as follows.
As shown in FIG. 15, a bias current Is flowing through the MR head is fixed to a prescribed value, and a relationship between a height H of the MR head and the life time is examined while changing the height H of the MR head. Thereafter, an MR head having a height H at which the life time of the MR head is longer than a desired life time is selectively used.
Also, as shown in FIG. 16, a bias current Is flowing through the MR head is fixed to a prescribed value, and a relationship between a height H of the MR head and an amplitude Viso of an output voltage waveform is examined while changing the height H of the MR head. Thereafter, an MR head having a height H at which the amplitude Viso of the output voltage waveform is longer than a desired amplitude is selectively used.
However, even though the bias current Is is determined according to one of the methods, a value of the bias current Is flowing though a particular MR head in which the MR resistance is low becomes too low, so that a high output cannot be obtained in the particular MR head. Also, because it is required to determine an upper limit of the MR resistance when the bias current Is is determined according to one of the methods, any MR head in which the MR resistance is higher than the upper limit cannot be used, so that many MR heads are uselessly manufactured in a large scale manufacturing.